Joining ceramic and metal parts has proven to be one of the critical technical challenges facing the material scientists fabricating devices used in high temperature electrochemical applications. The ability to join a metal part to a ceramic part, or a ceramic part to another ceramic part, theoretically provides an economical way to manufacturing complex ceramic components from inexpensive, simple-shaped ceramic parts, and to provide a hermetic seal between components consisting of dissimilar materials. However, while a number of joining techniques, such as glass joining and active metal brazing are currently used, each possesses some form of trade-off or exhibits some penalty in terms of joint properties, ease of processing, and/or cost.
As an alternative, a simple and economical joining technique referred to as reactive air brazing (RAB) has been recently developed and demonstrated for joining several different substrates. As described in J. S. Hardy, J. Y. Kim, K. S. Weil, “Joining Mixed Conducting Oxides Using An Air-Fired Electrically Conductive Braze,” J. Electrochem. Soc. Vol. 151, No. 8, pp. j43-j49 and U.S. patent application Ser. No. 10/334,346, now U.S. Pat. No. 7,055,733 RAB differs from conventional active metal brazing because RAB does not require the stringent atmosphere control normally associated with conventional active metal brazing. Instead, the RAB technique is conducted directly in air without the use of flux or reducing agents to promote wetting.
The braze filler materials of the RAB consist of two ingredients, a noble metal and an oxide compound. An oxide compound, which dissolves in a molten noble metal, is added to reactively modify the oxide faying surface and to help the remaining molten filler material wet on it. The resulting joint is adherent, ductile, and oxidation resistant. Due to the ductility and compliance of the noble metal, for example silver, this brazing can be used for high temperature electrochemical devices, even though there is a significant mismatch in the coefficient of thermal expansion (CTE) between silver (22.8 ppm/° C.) and typical ceramic components, such as yttria-stabilized zirconia (YSZ, 10.5 ppm/° C.)
One drawback of the RAB technique that has been identified in silver-copper oxide (Ag—CuO) based reactive air brazing systems for high temperature electrochemical applications relates to the propensity of silver to undergo a form of high-temperature embrittlement. This occurs due to the reaction of hydrogen diffused into the braze at one side and oxygen diffused into the braze at the other side when the silver-copper oxide braze is simultaneously exposed to a reducing atmosphere on one side and an oxidizing atmosphere on the other, as is typical in fuel cell applications. The present invention is a novel braze and method of forming a novel braze that addresses this problem, while preserving the advantages of silver-copper oxide (Ag—CuO) based reactive air brazing systems.